The invention relates generally to the field of capillary electrophoresis, and more particularly to materials and methods for suppressing electroendoosmotic flow and analyte-wall interactions during separation of biomolecules, especially polynucleotides, by capillary electrophoresis.
Capillary electrophoresis has been applied widely as an analytical technique because of several technical advantages: (i) capillaries have high surface-to-volume ratios which permit more efficient heat dissipation which, in turn, permit high electric fields to be used for more rapid separations; (ii) the technique requires minimal sample volumes; (iii) superior resolution of most analytes is attainable; and (iv) the technique is amenable to automation, e.g. Camilleri, editor, Capillary Electrophoresis: Theory and Practice (CRC Press, Boca Raton, 1993); and Grossman et al, editors, Capillary Electrophoresis (Academic Press, San Diego, 1992). Because of these advantages, there has been great interest in applying capillary electrophoresis to the separation of biomolecules, particularly in nucleic acid analysis. The need for rapid and accurate separation of nucleic acids, particularly deoxyribonucleic acid (DNA) arises in the analysis of polymerase chain reaction (PCR) products and DNA sequencing fragment analysis, e.g. Williams, Methods 4: 227-232 (19920; Drossman et al, Anal. Chem., 62: 900-903 (1990); Huang et al, Anal. Chem., 64: 2149-2154 (1992); and Swerdlow et al, Nucleic Acids Research, 18: 1415-1419 (1990).
Since the charge-to-frictional drag ratio is the same for different sized polynucleotides in free solution, electrophoretic separation requires the presence of a sieving medium. The initial sieving media of choice were gels, but problems of stability and manufacturability have led to the examination of non-gel liquid polymeric sieving media, such as linear polyacrylamide, hydroxyalkylcellulose, agarose, and cellulose acetate, and the like, e.g. Bode, Anal. Biochem., 83: 204-210 (1977); Bode, Anal. Biochem., 83: 364-371 (1977); Bode, Anal. Biochem., 92: 99-110 (1979); Hjerten et al, J. Liquid Chromatography, 12: 2471-2477 (1989); Grossman, U.S. Pat. No. 5,126,021; Zhu et al, U.S. Pat. No. 5,089,111; Tietz et al, Electrophoresis, 13: 614-616 (1992).
Another factor that complicates separations by capillary electrophoresis is the phenomena of electroendoosmosis. This phenomena, sometimes referred to as electroosmosis, is fluid flow in a capillary induced by an electrical field. It has impeded the application of capillary electrophoresis to situations where high resolution separations are required, such as in the analysis of DNA sequencing fragments. The phenomena arises in capillary electrophoresis when the inner wall of the capillary contains immobilized charges which cause the formation of a mobile layer of counter ions which, in turn, moves in the presence of an electrical field to create a bulk flow of liquid. Unfortunately, the magnitude of the electroendoosmotic flow can vary depending on a host of factors, including variation in the distribution of charges, selective adsorption of components of the analyte and/or separation medium, pH of the separation medium, and the like. Because this variability tends to reduce ones ability to resolve closely spaced bands analyte, many attempts have been made to directly or indirectly control such flow. The attempts have included covalent modification of the inner wall of the capillary to suppress charged groups, use of high viscosity polymers, adjustment of buffer pH and/or concentration, use of a gel separation medium covalently attached to the capillary wall, and the application of an electric field radial to the axis of the capillary, e.g. Hayes et al, Anal. Chem., 65: 2010-2013 (1993); Drossman et al (cited above); Hjerten, U.S. Pat. No. 4,680,201; Van Alstine et al, U.S. Pat. No. 4,690,749; Wiktorowicz et al, Electrophoresis, 11: 769-773 (1990); Belder et al, J. High Resolution Chromatography, 15: 686-693 (1992).
Most of these approaches have met with mixed success or have only been used in the separation of analytes quite different chemically from nucleic acids. In particular, the use of capillary gels for DNA separations have been hampered by manufacturing problems and problems of stability and reliability during use, e.g. Swerdlow et al, Electrophoresis, 13: 475-483 (1992).
In view of the strong scientific and industrial interest in being able to conveniently and accurately separate a variety of biomolecules, particularly polynucleotides, it would be desirable to have available a low viscosity electrophoretic separation medium capable of suppressing electroendoosmotic flow and of reducing analyte-wall interactions.
The invention relates to the use of uncharged water-soluble silica-adsorbing polymers to suppress electroendoosmotic flow and to reduce analyte-wall interactions in capillary electrophoresis. In one aspect of the invention, one or more of such polymers are employed as components of a separation medium for the separation of biomolecules, preferably polynucleotides, by capillary electrophoresis. Generally, such polymers are characterized by (i) water solubility over the temperature range between about 20xc2x0 C. to about 50xc2x0 C., (ii) concentration in a separation medium in the range between about 0.001% to about 10% (weight/volume), (iii) molecular weight in the range of about 5xc3x97103 to about 1xc3x97106 daltons, and (iv) absence of charged groups in an aqueous medium having pH in the range of about 6 to about 9. Preferably, such polymers of the invention are substantially non-hydroxylic. In one embodiment, polymers of the invention are selected from the group consisting of polyvinylactams, such as polyvinylpyrrolidone; N,N-disubstituted polyacrylamides; and N-substituted polyacrylamides. More preferably, such polymers of the invention are poly(N,N-dimethylacrylamide).
In accordance with the method of the invention, a sufficient amount of polymer adsorbs to the silica surface to establish a zone of high viscosity at the silica surface that impedes the movement of an electrical double layer under an electric field and that shields the analyte from the wall.
The invention includes methods of using the polymers of the invention to separate biomolecules, especially polynucleotides, by capillary electrophoresis; compositions comprising polymers of the invention for electrophoretically separating biomolecules in capillaries; and methods of using the separation medium of the invention for sequencing DNA.
The invention enhances the precision of biomolecule separation by electrophoresis in a capillary by dynamically suppressing electroendoosmotic flow and wall-analyte interactions through the adsorption of the uncharged polymers of the invention onto the surface of the capillary. Suppression is dynamic in the sense that throughout the separation process polymers of the invention adsorb and desorb from the surface of a capillary in equilibrium with polymer in solution in the separation medium. Thus, a constant degree of suppression is maintained not only during a separation run, but also from separation run to separation run.